memchr(), memchr_isr()

Updated: October 26, 2022

Find the first occurrence of a character in a buffer

Synopsis:

#include <string.h>

void* memchr( void* buf,
              int ch,
              size_t length );

void* memchr_isr( void* buf,
                  int ch,
                  size_t length );

Arguments:

buf
The buffer that you want to search.
ch
The character that you're looking for.
length
The number of bytes to search in the buffer.

Library:

libc

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

Description:

The memchr() function locates the first occurrence of ch (converted to an unsigned char) in the first length bytes of the buffer pointed to by buf.

The memchr_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 memory-copying operations, the specific target, and what other processes are doing. If memory 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 memchr() and memchr_isr() are safe to call from an interrupt handler.

Note: These functions give sensible results only if &buf[length - 1] is within the memory range allocated for buf. For example, consider the following code:
char buf[16];
char *p = memchr(buf, 'a', 100);

If the letter a doesn't appear within buf and if the memory at buf + 40 happens to be an a, then the call returns &buf[40], but that's outside the allocated buffer and in most circumstances would represent a bogus return value. If a page boundary is crossed between buf and buf + 100, then the function can result in the delivery of a fatal signal to the process. The only way to avoid these issues is ensure that &buf[length - 1] is within the region allocated for buf.

Returns:

A pointer to the located character, or NULL if ch couldn't be found.

Examples:

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

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

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

    where = (char *) memchr( buffer, 'x', 6 );
    if( where == NULL ) {
        printf( "'x' not found\n" );
    } else {
        printf( "%s\n", where );
    }

    where = (char *) memchr( buffer, 'r', 9 );
    if( where == NULL ) {
        printf( "'r' not found\n" );
    } else {
        printf( "%s\n", where );
    }
    
    return EXIT_SUCCESS;
}

produces the output:

'x' not found
rays

Environment variables:

LIBC_STRINGS
On certain targets, you can use this environment variable to select the implementation of memchr(). It doesn't affect the implementation of memchr_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.

Classification:

memchr() is ANSI, POSIX 1003.1; memchr_isr() is QNX Neutrino.

Safety:  
Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes