strsep(3)

✨ strsep(3) ✨

okay bestie, we're in the Library Functions Manual today and the star of the show is strsep, the string separate queen 👑 she lives in the Standard C library (libc, -lc) and honestly? she's that girl. let's get into it 💅

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🎀 NAME 🎀

strsep — string separate, periodt.

📚 LIBRARY 📚

Standard C library (libc, -lc). yeah she's in libc, she's not some obscure little side quest 🫶

🌸 SYNOPSIS 🌸

#include <string.h>

char *strsep(char **restrict stringp, const char *restrict delim);

Feature Test Macro Requirements for glibc (go peep feature_test_macros(7) for the full tea ☕):

• strsep():
  • Since glibc 2.19:
    • _DEFAULT_SOURCE
  • glibc 2.19 and earlier:
    • _BSD_SOURCE

🦋 DESCRIPTION 🦋

okay so here's the vibe. if *stringp is NULL, strsep() just returns NULL and does literally nothing else. no drama, no side quests, she said "not today" 💀

otherwise? she goes hunting for the first token in the string *stringp that's delimited by one of the bytes in the string delim. and the way she does it is iconic: this token gets terminated by overwriting the delimiter with a null byte ('\0'), and then *stringp gets updated to point past the token. so yeah, she's out here editing in place, main character energy only ✨

and if no delimiter was found at all? no worries babes — the token is just taken to be the entire string *stringp, and *stringp is made NULL. slay.

💖 RETURN VALUE 💖

strsep() returns a pointer to the token. which is, like, literally just the original value of *stringp. she gives you back what you gave her, but transformed 🦋

⭐ ATTRIBUTES ⭐

for an explanation of the terms used in this section, go check attributes(7) bestie.

• Interface: strsep()
  • Attribute: Thread safety
  • Value: MT-Safe

translation: she's MT-Safe, she can hang in a crowd, no cap 🫶

🌈 STANDARDS 🌈

BSD.

💜 HISTORY 💜

4.4BSD.

and not me telling you that strsep() was introduced as a replacement for strtok(3) 🌈 because strtok(3) literally cannot handle empty fields. that's the whole origin story. strtok said "no empty fields" and strsep said "bet" 💅

⚠️ CAVEATS ⚠️

be cautious when using this function, fr fr. if you do use it, note that: 👇

• the identity of the delimiting character is lost. like, gone. she overwrites it with a null byte, so that lil delimiter does not survive the encounter 😭

🔥 EXAMPLES 🔥

the program below is a port of the one found in strtok(3) — except this one doesn't discard multiple delimiters or empty tokens. so you get to see every single one, including the empty little guys 🙈

$ ./a.out 'a/bbb///cc;xxx:yyy:' ':;' '/'
1: a/bbb///cc
        --> a
        --> bbb
        -->
        -->
        --> cc
2: xxx
        --> xxx
3: yyy
        --> yyy
4:
        -->

🧚 Program source 🧚

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

int
main(int argc, char *argv[])
{
    char *token, *subtoken;

    if (argc != 4) {
        fprintf(stderr, "Usage: %s string delim subdelim\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    for (unsigned int j = 1; (token = strsep(&argv[1], argv[2])); j++) {
        printf("%u: %s\n", j, token);

        while ((subtoken = strsep(&token, argv[3])))
            printf("\t --> %s\n", subtoken);
    }

    exit(EXIT_SUCCESS);
}

👑 SEE ALSO 👑

string(3)

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and that's the whole page babes! this iconic lil number is from Linux man-pages 6.20, dated 2026-08-10, and yes she signs her name strsep(3) at the bottom like the queen she is 👑💖 go separate some strings, you got this 🫶✨