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Strspn

As explained in Modern C, Third Edition

strspn is a C standard-library function that returns the length of the initial segment containing only characters from a specified string.

strspn returns the length of the initial segment of one null-terminated string whose characters all occur in a second string. Include <string.h> and use its size_t result. section 8.5 section 6.1.5

Skipping leading whitespace

#include <stdio.h> #include <string.h> int main(void) { char const input[] = " \t\n42"; size_t const count = strspn(input, " \f\n\r\t\v"); printf("%zu\n", count); } The call returns 3: the initial space, tab, and newline belong to the allowed-character string. A parser can then continue at input + count, as the strtoul implementation does after skipping whitespace. section 8.5 section 6.1.5

Both arguments must be strings: each must be terminated by a zero character. Passing a character array without that terminator can make the function read beyond the array boundary and cause slow execution, a segmentation violation, or data corruption. section 6.1.5

Related behavior:

  • strcspn counts the initial characters that do not occur in the second string. section 8.5
  • Use memchr and explicit length information instead when the character array may not be null-terminated. section 6.1.5

How Modern C, Third Edition covers it

How deep it goes
Defines the initial-segment rule and prototype, then emphasizes null-termination requirements without discussing implementation internals.
Examples
Demonstrates skipping whitespace before numeric conversion and contrasts string scanning with memchr for unterminated arrays.
What it assumes
Assumes familiarity with C character arrays and teaches that string functions require zero-terminated strings.
What you can do after
Apply it via <string.h> to count an allowed-character prefix during parsing.
How it connects
Connects strspn with strcspn, strtoul, null-terminated strings, string functions, and bounded memory scanning.
Cover of Modern C, Third Edition
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Related concepts

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