strchr(), strchr_isr()

Updated: April 19, 2023

Find the first occurrence of a character in a string


#include <string.h>

char* strchr( char* s, 
              int c );

char* strchr_isr( char* s, 
                  int c );


The string that you want to search.
The character that you're looking for.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The strchr() function finds the first occurrence of c (converted to a char) in the string pointed to by s. The terminating NUL character is considered to be part of the string.

The strchr_isr() function does the same thing, but avoids using special-purpose registers (e.g., sse2) that incur additional cost when their state is saved during context switches. The relative performance of a process using this first or second function can be better or worse depending on the frequency and sizes of string-copying operations, the specific target, and what other processes are doing. If string copying does not play a major role in the process's overall performance, which function is faster isn't so important. Otherwise, developers are strongly encouraged to do their own testing and select the correct function.

In this release, both strchr() and strchr_isr() are safe to call from an interrupt handler.


A pointer to the located character, or NULL if c doesn't occur in the string.


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

int main( void )
    char buffer[80];
    char* where;

    strcpy( buffer, "video x-rays" );

    where = strchr( buffer, 'x' );

    if( where == NULL ) {
        printf( "'x' not found\n" );
    } else {
        printf( "'x' found: %s\n", where );

    return EXIT_SUCCESS;

Environment variables:

On certain targets, you can use this environment variable to select the implementation of strchr(). It doesn't affect the implementation of strchr_isr(). The value is one of the strings given below.
  • for AArch64 targets:
    • aarch64_neon — optimized for AARCH64 targets using NEON
    • generic — the default
  • for ARMv7 targets:
    • cortex_a9 — optimized for the ARM Cortex-A9 processor; assumes that no unaligned access is supported
    • cortex_a9_aligned — optimized for ARM Cortex-A9; requires that unaligned memory access be enabled on the platform. If memory access is misaligned, this implementation falls back to the NEON version.
    • cortex_a9_neon — optimized for ARM Cortex-A9 using NEON
    • generic — the default
    • krait — optimized for the Qualcomm Krait CPU
    • krait_neon — optimized for Qualcomm Krait using NEON

Processes that register ISRs shouldn't use the NEON versions.


strchr() is ANSI, POSIX 1003.1; strchr_isr() is QNX Neutrino.

Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes