# Strings

In C, a string is not an intrinsic type. A C-string is the convention to have a one-dimensional array of characters which is terminated by a null-character, by a '\0'.

This means that a C-string with a content of "abc" will have four characters 'a', 'b', 'c' and '\0'.

See the basic introduction to strings (opens new window) example.

# Tokenisation: strtok(), strtok_r() and strtok_s()

The function strtok breaks a string into a smaller strings, or tokens, using a set of delimiters.

#include <stdio.h>
#include <string.h>

int main(void)
{
    int toknum = 0;
    char src[] = "Hello,, world!";
    const char delimiters[] = ", !";
    char *token = strtok(src, delimiters);
    while (token != NULL)
    {
        printf("%d: [%s]\n", ++toknum, token);
        token = strtok(NULL, delimiters);
    }
    /* source is now "Hello\0, world\0\0" */
}

Output:

1: [Hello]
2: [world]

The string of delimiters may contain one or more delimiters and different delimiter strings may be used with each call to strtok.

Calls to strtok to continue tokenizing the same source string should not pass the source string again, but instead pass NULL as the first argument. If the same source string is passed then the first token will instead be re-tokenized. That is, given the same delimiters, strtok would simply return the first token again.

Note that as strtok does not allocate new memory for the tokens, it modifies the source string. That is, in the above example, the string src will be manipulated to produce the tokens that are referenced by the pointer returned by the calls to strtok. This means that the source string cannot be const (so it can't be a string literal). It also means that the identity of the delimiting byte is lost (i.e. in the example the "," and "!" are effectively deleted from the source string and you cannot tell which delimiter character matched).

Note also that multiple consecutive delimiters in the source string are treated as one; in the example, the second comma is ignored.

strtok is neither thread safe nor re-entrant because it uses a static buffer while parsing. This means that if a function calls strtok, no function that it calls while it is using strtok can also use strtok, and it cannot be called by any function that is itself using strtok.

An example that demonstrates the problems caused by the fact that strtokis not re-entrant is as follows:

char src[] = "1.2,3.5,4.2";
char *first = strtok(src, ","); 

do 
{
    char *part;
    /* Nested calls to strtok do not work as desired */
    printf("[%s]\n", first);
    part = strtok(first, ".");
    while (part != NULL)
    {
        printf(" [%s]\n", part);
        part = strtok(NULL, ".");
    }
} while ((first = strtok(NULL, ",")) != NULL);

Output:

[1.2]
 [1]
 [2]

The expected operation is that the outer do while loop should create three tokens consisting of each decimal number string ("1.2", "3.5", "4.2"), for each of which the strtok calls for the inner loop should split it into separate digit strings ("1", "2", "3", "5", "4", "2").

However, because strtok is not re-entrant, this does not occur. Instead the first strtok correctly creates the "1.2\0" token, and the inner loop correctly creates the tokens "1" and "2". But then the strtok in the outer loop is at the end of the string used by the inner loop, and returns NULL immediately. The second and third substrings of the src array are not analyzed at all.

The standard C libraries do not contain a thread-safe or re-entrant version but some others do, such as POSIX' strtok_r (opens new window). Note that on MSVC the strtok equivalent, strtok_s is thread-safe.

C11 has an optional part, Annex K, that offers a thread-safe and re-entrant version named strtok_s. You can test for the feature with __STDC_LIB_EXT1__. This optional part is not widely supported.

The strtok_s function differs from the POSIX strtok_r function by guarding against storing outside of the string being tokenized, and by checking runtime constraints. On correctly written programs, though, the strtok_s and strtok_r behave the same.

Using strtok_s with the example now yields the correct response, like so:

/* you have to announce that you want to use Annex K */ 
#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>

#ifndef __STDC_LIB_EXT1__
# error "we need strtok_s from Annex K"
#endif

char src[] = "1.2,3.5,4.2";  
char *next = NULL;
char *first = strtok_s(src, ",", &next);

do 
{
    char *part;
    char *posn;

    printf("[%s]\n", first);
    part = strtok_s(first, ".", &posn);
    while (part != NULL)
    {
        printf(" [%s]\n", part);
        part = strtok_s(NULL, ".", &posn);
    }
} 
while ((first = strtok_s(NULL, ",", &next)) != NULL);

And the output will be:

[1.2]
 [1]
 [2]
[3.5]
 [3]
 [5]
[4.2]
 [4]
 [2]

# String literals

String literals represent null-terminated, static-duration (opens new window) arrays of char. Because they have static storage duration, a string literal or a pointer to the same underlying array can safely be used in several ways that a pointer to an automatic array cannot. For example, returning a string literal from a function has well-defined behavior:

const char *get_hello() {
    return "Hello, World!";  /* safe */
}

For historical reasons, the elements of the array corresponding to a string literal are not formally const. Nevertheless, any attempt to modify them has undefined behavior (opens new window). Typically, a program that attempts to modify the array corresponding to a string literal will crash or otherwise malfunction.

char *foo = "hello";
foo[0] = 'y';  /* Undefined behavior - BAD! */

Where a pointer points to a string literal -- or where it sometimes may do -- it is advisable to declare that pointer's referent const to avoid engaging such undefined behavior accidentally.

const char *foo = "hello";
/* GOOD: can't modify the string pointed to by foo */

On the other hand, a pointer to or into the underlying array of a string literal is not itself inherently special; its value can freely be modified to point to something else:

char *foo = "hello";
foo = "World!"; /* OK - we're just changing what foo points to */

Furthermore, although initializers for char arrays can have the same form as string literals, use of such an initializer does not confer the characteristics of a string literal on the initialized array. The initializer simply designates the length and initial contents of the array. In particular, the elements are modifiable if not explicitly declared const:

char foo[] = "hello";
foo[0] = 'y';  /* OK! */

# Calculate the Length: strlen()

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv) 
{
    /* Exit if no second argument is found. */
    if (argc != 2) 
    {
        puts("Argument missing.");
        return EXIT_FAILURE;
    }

    size_t len = strlen(argv[1]);
    printf("The length of the second argument is %zu.\n", len);

    return EXIT_SUCCESS;
}

This program computes the length of its second input argument and stores the result in len. It then prints that length to the terminal. For example, when run with the parameters program_name "Hello, world!", the program will output The length of the second argument is 13. because the string Hello, world! is 13 characters long.

strlen counts all the bytes from the beginning of the string up to, but not including, the terminating NUL character, '\0'. As such, it can only be used when the string is guaranteed to be NUL-terminated.

Also keep in mind that if the string contains any Unicode characters, strlen will not tell you how many characters are in the string (since some characters may be multiple bytes long). In such cases, you need to count the characters (i.e., code units) yourself. Consider the output of the following example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) 
{
    char asciiString[50] = "Hello world!";
    char utf8String[50] = "Γειά σου Κόσμε!"; /* "Hello World!" in Greek */

    printf("asciiString has %zu bytes in the array\n", sizeof(asciiString));
    printf("utf8String has %zu bytes in the array\n", sizeof(utf8String));
    printf("\"%s\" is %zu bytes\n", asciiString, strlen(asciiString));
    printf("\"%s\" is %zu bytes\n", utf8String, strlen(utf8String));
}

Output:

asciiString has 50 bytes in the array
utf8String has 50 bytes in the array
"Hello world!" is 12 bytes
"Γειά σου Κόσμε!" is 27 bytes

# Basic introduction to strings

In C, a string is a sequence of characters that is terminated by a null character ('\0').

We can create strings using string literals, which are sequences of characters surrounded by double quotation marks; for example, take the string literal "hello world". String literals are automatically null-terminated.

We can create strings using several methods. For instance, we can declare a char * and initialize it to point to the first character of a string:

char * string = "hello world";

When initializing a char * to a string constant as above, the string itself is usually allocated in read-only data; string is a pointer to the first element of the array, which is the character 'h'.

Since the string literal is allocated in read-only memory, it is non-modifiable¹. Any attempt to modify it will lead to undefined behaviour (opens new window), so it's better to add const to get a compile-time error like this

char const * string = "hello world";

It has similar effect² as

char const string_arr[] = "hello world";

To create a modifiable string, you can declare a character array and initialize its contents using a string literal, like so:

char modifiable_string[] = "hello world";

This is equivalent to the following:

char modifiable_string[] = {'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '\0'};

Since the second version uses brace-enclosed initializer, the string is not automatically null-terminated unless a '\0' character is included explicitly in the character array usually as its last element.

_{1 Non-modifiable implies that the characters in the string literal can't be modified, but remember that the pointer string can be modified (can point somewhere else or can be incremented or decremented).}

_{2 Both strings have similar effect in a sense that characters of both strings can't be modified. It should be noted that string is a pointer to char and it is a modifiable l-value (opens new window) so it can be incremented or point to some other location while the array string_arr is a non-modifiable l-value, it can't be modified.}

# Copying strings

# Pointer assignments do not copy strings

You can use the = operator to copy integers, but you cannot use the = operator to copy strings in C. Strings in C are represented as arrays of characters with a terminating null-character, so using the = operator will only save the address (pointer) of a string.

#include <stdio.h>

int main(void) {
    int a = 10, b;
    char c[] = "abc", *d;

    b = a; /* Integer is copied */
    a = 20; /* Modifying a leaves b unchanged - b is a 'deep copy' of a */
    printf("%d %d\n", a, b); /* "20 10" will be printed */

    d = c; 
    /* Only copies the address of the string - 
    there is still only one string stored in memory */
    
    c[1] = 'x';
    /* Modifies the original string - d[1] = 'x' will do exactly the same thing */

    printf("%s %s\n", c, d); /* "axc axc" will be printed */

    return 0;
}

The above example compiled because we used char *d rather than char d[3]. Using the latter would cause a compiler error. You cannot assign to arrays in C.

#include <stdio.h>

int main(void) {
    char a[] = "abc";
    char b[8];

    b = a; /* compile error */
    printf("%s\n", b);

    return 0;
}

# Copying strings using standard functions

# `strcpy()`

To actually copy strings, strcpy() (opens new window) function is available in string.h. Enough space must be allocated for the destination before copying.

#include <stdio.h>
#include <string.h>

int main(void) {
    char a[] = "abc";
    char b[8];

    strcpy(b, a); /* think "b special equals a" */
    printf("%s\n", b); /* "abc" will be printed */

    return 0;
}

# `snprintf()`

To avoid buffer overrun, snprintf() (opens new window) may be used. It is not the best solution performance-wise since it has to parse the template string, but it is the only buffer limit-safe function for copying strings readily-available in standard library, that can be used without any extra steps.

#include <stdio.h>
#include <string.h>

int main(void) {
    char a[] = "012345678901234567890";
    char b[8];

#if 0
    strcpy(b, a); /* causes buffer overrun (undefined behavior), so do not execute this here! */
#endif

    snprintf(b, sizeof(b), "%s", a); /* does not cause buffer overrun */
    printf("%s\n", b); /* "0123456" will be printed */

    return 0;
}

# `strncat()`

A second option, with better performance, is to use strncat() (opens new window) (a buffer overflow checking version of strcat()) - it takes a third argument that tells it the maximum number of bytes to copy:

char dest[32];

dest[0] = '\0';
strncat(dest, source, sizeof(dest) - 1);
    /* copies up to the first (sizeof(dest) - 1) elements of source into dest,
    then puts a \0 on the end of dest */

Note that this formulation use sizeof(dest) - 1; this is crucial because strncat() always adds a null byte (good), but doesn't count that in the size of the string (a cause of confusion and buffer overwrites).

Also note that the alternative — concatenating after a non-empty string — is even more fraught. Consider:

char dst[24] = "Clownfish: ";
char src[] = "Marvin and Nemo";
size_t len = strlen(dst);

strncat(dst, src, sizeof(dst) - len - 1);
printf("%zu: [%s]\n", strlen(dst), dst);

The output is:

23: [Clownfish: Marvin and N]

Note, though, that the size specified as the length was not the size of the destination array, but the amount of space left in it, not counting the terminal null byte. This can cause big overwriting problems. It is also a bit wasteful; to specify the length argument correctly, you know the length of the data in the destination, so you could instead specify the address of the null byte at the end of the existing content, saving strncat() from rescanning it:


   strcpy(dst, "Clownfish: ");
    assert(len < sizeof(dst) - 1);
    strncat(dst + len, src, sizeof(dst) - len - 1);
    printf("%zu: [%s]\n", strlen(dst), dst);

This produces the same output as before, but strncat() doesn't have to scan over the existing content of dst before it starts copying.

# `strncpy()`

The last option is the strncpy() (opens new window) function. Although you might think it should come first, it is a rather deceptive function that has two main gotchas:

If copying via strncpy() hits the buffer limit, a terminating null-character won't be written.
strncpy() always completely fills the destination, with null bytes if necessary.

(Such quirky implementation is historical and was initially intended for handling UNIX file names (opens new window))

The only correct way to use it is to manually ensure null-termination:

strncpy(b, a, sizeof(b)); /* the third parameter is destination buffer size */
b[sizeof(b)/sizeof(*b) - 1] = '\0'; /* terminate the string */
printf("%s\n", b); /* "0123456" will be printed */

Even then, if you have a big buffer it becomes very inefficient to use strncpy() because of additional null padding.

# Iterating Over the Characters in a String

If we know the length of the string, we can use a for loop to iterate over its characters:

char * string = "hello world"; /* This 11 chars long, excluding the 0-terminator. */
size_t i = 0;
for (; i < 11; i++) {
    printf("%c\n", string[i]);    /* Print each character of the string. */
}

Alternatively, we can use the standard function strlen() to get the length of a string if we don't know what the string is:

size_t length = strlen(string);
size_t i = 0; 
for (; i < length; i++) {
    printf("%c\n", string[i]);    /* Print each character of the string. */
}

Finally, we can take advantage of the fact that strings in C are guaranteed to be null-terminated (which we already did when passing it to strlen() in the previous example 😉). We can iterate over the array regardless of its size and stop iterating once we reach a null-character:

size_t i = 0;
while (string[i] != '\0') {       /* Stop looping when we reach the null-character. */
    printf("%c\n", string[i]);    /* Print each character of the string. */
    i++;
}

# Creating Arrays of Strings

An array of strings can mean a couple of things:

An array whose elements are char *s
An array whose elements are arrays of chars

We can create an array of character pointers like so:

char * string_array[] = {
    "foo",
    "bar",
    "baz"
};

Remember: when we assign string literals to char *, the strings themselves are allocated in read-only memory. However, the array string_array is allocated in read/write memory. This means that we can modify the pointers in the array, but we cannot modify the strings they point to.

In C, the parameter to main argv (the array of command-line arguments passed when the program was run) is an array of char *: char * argv[].

We can also create arrays of character arrays. Since strings are arrays of characters, an array of strings is simply an array whose elements are arrays of characters:

char modifiable_string_array_literals[][4] = {
    "foo",
    "bar",
    "baz"
};

This is equivalent to:

char modifiable_string_array[][4] = {
    {'f', 'o', 'o', '\0'},
    {'b', 'a', 'r', '\0'},
    {'b', 'a', 'z', '\0'}
};

Note that we specify 4 as the size of the second dimension of the array; each of the strings in our array is actually 4 bytes since we must include the null-terminating character.

# Convert Strings to Number: atoi(), atof() (dangerous, don't use them)

Warning: The functions atoi, atol, atoll and atof are inherently unsafe, because: If the value of the result cannot be represented, the behavior is undefined. (opens new window) (7.20.1p1)

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char** argv)
{
    int val;
    if (argc < 2)
    {
        printf("Usage: %s <integer>\n", argv[0]);
        return 0;
    }

    val = atoi(argv[1]);

    printf("String value = %s, Int value = %d\n", argv[1], val);

    return 0;
}

When the string to be converted is a valid decimal integer that is in range, the function works:

$ ./atoi 100
String value = 100, Int value = 100
$ ./atoi 200
String value = 200, Int value = 200

For strings that start with a number, followed by something else, only the initial number is parsed:

$ ./atoi 0x200
0
$ ./atoi 0123x300
123

In all other cases, the behavior is undefined:

$ ./atoi hello
Formatting the hard disk...

Because of the ambiguities above and this undefined behavior, the atoi family of functions should never be used.

To convert to long int, use strtol() instead of atol().
To convert to double, use strtod() instead of atof().
To convert to long long int, use strtoll() instead of atoll().

# string formatted data read/write

Write formatted data to string

int sprintf ( char * str, const char * format, ... );

use sprintf function to write float data to string.

#include <stdio.h>
int main ()
{
  char buffer [50];
  double PI = 3.1415926;
  sprintf (buffer, "PI = %.7f", PI);
  printf ("%s\n",buffer);
  return 0;
}

Read formatted data from string

int sscanf ( const char * s, const char * format, ...);

use sscanffunction to parse formatted data.

#include <stdio.h>
int main ()
{
  char sentence []="date : 06-06-2012";
  char str [50];
  int year;
  int month;
  int day;
  sscanf (sentence,"%s : %2d-%2d-%4d", str, &day, &month, &year);
  printf ("%s -> %02d-%02d-%4d\n",str, day, month, year);
  return 0;
}

# Copy and Concatenation: strcpy(), strcat()

#include <stdio.h>
#include <string.h>

int main(void)
{
  /* Always ensure that your string is large enough to contain the characters
   * and a terminating NUL character ('\0')!
   */
  char mystring[10];

  /* Copy "foo" into `mystring`, until a NUL character is encountered. */
  strcpy(mystring, "foo");
  printf("%s\n", mystring);

  /* At this point, we used 4 chars of `mystring`, the 3 characters of "foo",
   * and the NUL terminating byte.
   */

  /* Append "bar" to `mystring`. */
  strcat(mystring, "bar");
  printf("%s\n", mystring);

  /* We now use 7 characters of `mystring`: "foo" requires 3, "bar" requires 3
   * and there is a terminating NUL character ('\0') at the end.
   */

  /* Copy "bar" into `mystring`, overwriting the former contents. */
  strcpy(mystring, "bar");
  printf("%s\n", mystring);

  return 0;
}

Outputs:

foo
foobar
bar

If you append to or from or copy from an existing string, ensure it is NUL-terminated!

String literals (e.g. "foo") will always be NUL-terminated by the compiler.

# Comparsion: strcmp(), strncmp(), strcasecmp(), strncasecmp()

The strcase*-functions are not Standard C, but a POSIX extension.

The strcmp function lexicographically compare two null-terminated character arrays. The functions return a negative value if the first argument appears before the second in lexicographical order, zero if they compare equal, or positive if the first argument appears after the second in lexicographical order.

#include <stdio.h>
#include <string.h>

void compare(char const *lhs, char const *rhs)
{
    int result = strcmp(lhs, rhs); // compute comparison once
    if (result < 0) {
        printf("%s comes before %s\n", lhs, rhs);
    } else if (result == 0) {
        printf("%s equals %s\n", lhs, rhs);
    } else { // last case: result > 0
        printf("%s comes after %s\n", lhs, rhs);
    }
}

int main(void)
{
    compare("BBB", "BBB");
    compare("BBB", "CCCCC");
    compare("BBB", "AAAAAA");
    return 0;
}

Outputs:

BBB equals BBB
BBB comes before CCCCC
BBB comes after AAAAAA

As strcmp, strcasecmp function also compares lexicographically its arguments after translating each character to its lowercase correspondent:

#include <stdio.h>
#include <string.h>

void compare(char const *lhs, char const *rhs)
{
    int result = strcasecmp(lhs, rhs); // compute case-insensitive comparison once
    if (result < 0) {
        printf("%s comes before %s\n", lhs, rhs);
    } else if (result == 0) {
        printf("%s equals %s\n", lhs, rhs);
    } else { // last case: result > 0
        printf("%s comes after %s\n", lhs, rhs);
    }
}

int main(void)
{
    compare("BBB", "bBB");
    compare("BBB", "ccCCC");
    compare("BBB", "aaaaaa");
    return 0;
}

Outputs:

BBB equals bBB
BBB comes before ccCCC
BBB comes after aaaaaa

strncmp and strncasecmp compare at most n characters:

#include <stdio.h>
#include <string.h>

void compare(char const *lhs, char const *rhs, int n)
{
    int result = strncmp(lhs, rhs, n); // compute comparison once
    if (result < 0) {
        printf("%s comes before %s\n", lhs, rhs);
    } else if (result == 0) {
        printf("%s equals %s\n", lhs, rhs);
    } else { // last case: result > 0
        printf("%s comes after %s\n", lhs, rhs);
    }
}

int main(void)
{
    compare("BBB", "Bb", 1);
    compare("BBB", "Bb", 2);
    compare("BBB", "Bb", 3);
    return 0;
}

Outputs:

BBB equals Bb
BBB comes before Bb
BBB comes before Bb

# Find first/last occurrence of a specific character: strchr(), strrchr()

The strchr and strrchr functions find a character in a string, that is in a NUL-terminated character array. strchr return a pointer to the first occurrence and strrchr to the last one.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    char toSearchFor = 'A';

    /* Exit if no second argument is found. */
    if (argc != 2)
    {
        printf("Argument missing.\n");
        return EXIT_FAILURE;
    }

    {
        char *firstOcc = strchr(argv[1], toSearchFor);
        if (firstOcc != NULL) 
        {
            printf("First position of %c in %s is %td.\n", 
              toSearchFor, argv[1], firstOcc-argv[1]); /* A pointer difference's result 
                                     is a signed integer and uses the length modifier 't'. */
        }
        else
        {
            printf("%c is not in %s.\n", toSearchFor, argv[1]);
        }
    }

    {
        char *lastOcc = strrchr(argv[1], toSearchFor);
        if (lastOcc != NULL)
        {
            printf("Last position of %c in %s is %td.\n",
              toSearchFor, argv[1], lastOcc-argv[1]);
        }
    }

    return EXIT_SUCCESS;
}

Outputs (after having generate an executable named pos):

$ ./pos AAAAAAA
First position of A in AAAAAAA is 0.
Last position of A in AAAAAAA is 6.
$ ./pos BAbbbbbAccccAAAAzzz
First position of A in BAbbbbbAccccAAAAzzz is 1.
Last position of A in BAbbbbbAccccAAAAzzz is 15.
$  ./pos qwerty             
A is not in qwerty.

One common use for strrchr is to extract a file name from a path. For example to extract myfile.txt from C:\Users\eak\myfile.txt:

char *getFileName(const char *path)
{
    char *pend;

    if ((pend = strrchr(path, '\')) != NULL)
        return pend + 1;

    return NULL;
}

# strspn and strcspn

Given a string, strspn calculates the length of the initial substring (span) consisting solely of a specific list of characters. strcspn is similar, except it calculates the length of the initial substring consisting of any characters except those listed:

/*
  Provided a string of "tokens" delimited by "separators", print the tokens along
  with the token separators that get skipped.
*/
#include <stdio.h>
#include <string.h>

int main(void)
{
    const char sepchars[] = ",.;!?";
    char foo[] = ";ball call,.fall gall hall!?.,";
    char *s;
    int n;

    for (s = foo; *s != 0; /*empty*/) {
        /* Get the number of token separator characters. */
        n = (int)strspn(s, sepchars);

        if (n > 0)
            printf("skipping separators: << %.*s >> (length=%d)\n", n, s, n);

        /* Actually skip the separators now. */
        s += n;

        /* Get the number of token (non-separator) characters. */
        n = (int)strcspn(s, sepchars);

        if (n > 0)
            printf("token found: << %.*s >> (length=%d)\n", n, s, n);

        /* Skip the token now. */
        s += n;
    }

    printf("== token list exhausted ==\n");

    return 0;
}

Analogous functions using wide-character strings are wcsspn and wcscspn; they're used the same way.

# Safely convert Strings to Number: strtoX functions

Since C99 the C library has a set of safe conversion functions that interpret a string as a number. Their names are of the form strtoX, where X is one of l, ul, d, etc to determine the target type of the conversion

double strtod(char const* p, char** endptr);
long double strtold(char const* p, char** endptr);

They provide checking that a conversion had an over- or underflow:

double ret = strtod(argv[1], 0); /* attempt conversion */

/* check the conversion result. */
if ((ret == HUGE_VAL || ret == -HUGE_VAL) && errno == ERANGE) 
    return;  /* numeric overflow in in string */
else if (ret == HUGE_VAL && errno == ERANGE) 
    return; /* numeric underflow in in string */

/* At this point we know that everything went fine so ret may be used */

If the string in fact contains no number at all, this usage of strtod returns 0.0.

If this is not satisfactory, the additional parameter endptr can be used. It is a pointer to pointer that will be pointed to the end of the detected number in the string. If it is set to 0, as above, or NULL, it is simply ignored.

This endptr parameter provides indicates if there has been a successful conversion and if so, where the number ended:

char *check = 0;
double ret = strtod(argv[1], &check); /* attempt conversion */

/* check the conversion result. */
if (argv[1] == check) 
    return; /* No number was detected in string */
else if ((ret == HUGE_VAL || ret == -HUGE_VAL) && errno == ERANGE) 
    return; /* numeric overflow in in string */
else if (ret == HUGE_VAL && errno == ERANGE) 
    return; /* numeric underflow in in string */

/* At this point we know that everything went fine so ret may be used */

There are analogous functions to convert to the wider integer types:

long strtol(char const* p, char** endptr, int nbase);
long long strtoll(char const* p, char** endptr, int nbase);
unsigned long strtoul(char const* p, char** endptr, int nbase);
unsigned long long strtoull(char const* p, char** endptr, int nbase);

These functions have a third parameter nbase that holds the number base in which the number is written.

long a = strtol("101",   0, 2 ); /* a = 5L */
long b = strtol("101",   0, 8 ); /* b = 65L */
long c = strtol("101",   0, 10); /* c = 101L */
long d = strtol("101",   0, 16); /* d = 257L */
long e = strtol("101",   0, 0 ); /* e = 101L */
long f = strtol("0101",  0, 0 ); /* f = 65L */
long g = strtol("0x101", 0, 0 ); /* g = 257L */

The special value 0 for nbase means the string is interpreted in the same way as number literals are interpreted in a C program: a prefix of 0x corresponds to a hexadecimal representation, otherwise a leading 0 is octal and all other numbers are seen as decimal.

Thus the most practical way to interpret a command-line argument as a number would be

int main(int argc, char* argv[] {
    if (argc < 1)
        return EXIT_FAILURE; /* No number given. */

    /* use strtoull because size_t may be wide */
    size_t mySize = strtoull(argv[1], 0, 0);

    /* then check conversion results. */

     ...

    return EXIT_SUCCESS;
}

This means that the program can be called with a parameter in octal, decimal or hexadecimal.

# strstr

/* finds the next instance of needle in haystack 
   zbpos: the zero-based position to begin searching from
   haystack: the string to search in
   needle: the string that must be found
   returns the next match of `needle` in `haystack`, or -1 if not found
*/
int findnext(int zbpos, const char *haystack, const char *needle)
{
    char *p; 

    if (((p = strstr(haystack + zbpos, needle)) != NULL)
        return p - haystack;

    return -1;
}

strstr searches the haystack (first) argument for the string pointed to by needle. If found, strstr returns the address of the occurrence. If it could not find needle, it returns NULL. We use zbpos so that we don't keep finding the same needle over and over again. In order to skip the first instance, we add an offset of zbpos. A Notepad clone might call findnext like this, in order to implement its "Find Next" dialogue:

/*
    Called when the user clicks "Find Next"
    doc: The text of the document to search
    findwhat: The string to find
*/
void onfindnext(const char *doc, const char *findwhat)
{
    static int i;

    if ((i = findnext(i, doc, findwhat)) != -1)
        /* select the text starting from i and ending at i + strlen(findwhat) */
    else
        /* display a message box saying "end of search" */
}

# Zeroing out a string

You can call memset to zero out a string (or any other memory block).

Where str is the string to zero out, and n is the number of bytes in the string.

#include <stdlib.h> /* For EXIT_SUCCESS */
#include <stdio.h>
#include <string.h>


int main(void)
{
  char str[42] = "fortytwo";
  size_t n = sizeof str; /* Take the size not the length. */

  printf("'%s'\n", str);

  memset(str, '\0', n);

  printf("'%s'\n", str);

  return EXIT_SUCCESS;
}

Prints:

'fortytwo'
''

Another example:

#include <stdlib.h> /* For EXIT_SUCCESS */
#include <stdio.h>
#include <string.h>


#define FORTY_STR "forty"
#define TWO_STR "two"

int main(void)
{
  char str[42] = FORTY_STR TWO_STR;
  size_t n = sizeof str; /* Take the size not the length. */
  char * point_to_two = strstr(str, TWO_STR);

  printf("'%s'\n", str);

  memset(point_to_two, '\0', n);

  printf("'%s'\n", str);

  memset(str, '\0', n);

  printf("'%s'\n", str);

  return EXIT_SUCCESS;
}

Prints:

'fortytwo'
'forty'
''

# Syntax

char str1[] = "Hello, world!"; /* Modifiable */
char str2[14] = "Hello, world!"; /* Modifiable */
char* str3 = "Hello, world!"; /* Non-modifiable*/

← Boolean Literals for numbers, characters and strings →

# Strings

# Tokenisation: strtok(), strtok_r() and strtok_s()

# String literals

# Calculate the Length: strlen()

# Basic introduction to strings

# Copying strings

# Pointer assignments do not copy strings

# Copying strings using standard functions

# strcpy()

# snprintf()

# strncat()

# strncpy()

# Iterating Over the Characters in a String

# Creating Arrays of Strings

# Convert Strings to Number: atoi(), atof() (dangerous, don't use them)

# string formatted data read/write

# Copy and Concatenation: strcpy(), strcat()

# Comparsion: strcmp(), strncmp(), strcasecmp(), strncasecmp()

# Find first/last occurrence of a specific character: strchr(), strrchr()

# strspn and strcspn

# Safely convert Strings to Number: strtoX functions

# strstr

# Zeroing out a string

# Syntax

# `strcpy()`

# `snprintf()`

# `strncat()`

# `strncpy()`