memchr

Your mission

Implement gm_memchr, your own memchr, which scans the first n bytes of a memory area for a given byte and returns a pointer to the first match, or NULL if it is absent.

See it in action

byte index of gm_memchr("docker", 'r', 6)

Expected result

Your checklist

For your own tracking. Tests grade your code.

Look at exactly n bytes starting at s, one at a time, this works on raw memory, not strings, so never stop early on a \0. A matching byte can sit past an embedded NUL as long as it is within the first n bytes.
Convert c to an unsigned char before comparing: the search byte is the low 8 bits of c, so a value like A + 256 must still match a stored A.
Return a pointer to the first matching byte. Return NULL when no match is found in the first n bytes, including when n is 0.
Do not write to the input buffer, and do not call any library function.

More examples

gm_memchr("zsh prompt", 'X', 10)

Expected result

null

Constraints

Expected files

memchr.c

Allowed functions

None. Write every helper yourself.

Allowed headers

<stddef.h>defines size_t

libc

Standard C library

Implements the contract of a function from the standard C library.

memchr

Bytes and raw memory

Your mission

Implement gm_memchr, your own memchr, which scans the first n bytes of a memory area for a given byte and returns a pointer to the first match, or NULL if it is absent.

See it in action

byte index of gm_memchr("docker", 'r', 6)

Expected result

Your checklist

For your own tracking. Tests grade your code.

Look at exactly n bytes starting at s, one at a time, this works on raw memory, not strings, so never stop early on a \0. A matching byte can sit past an embedded NUL as long as it is within the first n bytes.
Convert c to an unsigned char before comparing: the search byte is the low 8 bits of c, so a value like A + 256 must still match a stored A.
Return a pointer to the first matching byte. Return NULL when no match is found in the first n bytes, including when n is 0.
Do not write to the input buffer, and do not call any library function.

More examples

gm_memchr("zsh prompt", 'X', 10)

Expected result

null

Constraints

Expected files

memchr.c

Allowed functions

None. Write every helper yourself.

Allowed headers

<stddef.h>defines size_t

libc

Standard C library

Implements the contract of a function from the standard C library.

Tests

Run the tests to grade your code

Build your code, then check each test here. Open a result to inspect its output.

Build

01Match at the first byteWaiting

02Match in the middleWaiting

03Match at the last byte within nWaiting

04Byte absent returns NULLWaiting

05Match found past an embedded NULWaiting

06Stops at n, ignores later bytesWaiting

07c is cast to unsigned charWaiting

08Zero length returns NULLWaiting

memchr

Your mission

Implement gm_memchr, your own memchr, which scans the first n bytes of a memory area for a given byte and returns a pointer to the first match, or NULL if it is absent.

See it in action

byte index of gm_memchr("docker", 'r', 6)

Expected result

Your checklist

For your own tracking. Tests grade your code.

Look at exactly n bytes starting at s, one at a time, this works on raw memory, not strings, so never stop early on a \0. A matching byte can sit past an embedded NUL as long as it is within the first n bytes.
Convert c to an unsigned char before comparing: the search byte is the low 8 bits of c, so a value like A + 256 must still match a stored A.
Return a pointer to the first matching byte. Return NULL when no match is found in the first n bytes, including when n is 0.
Do not write to the input buffer, and do not call any library function.

More examples

gm_memchr("zsh prompt", 'X', 10)

Expected result

null

Constraints

Expected files

memchr.c

Allowed functions

None. Write every helper yourself.

Allowed headers

<stddef.h>defines size_t

libc

Standard C library

Implements the contract of a function from the standard C library.

memchr

Bytes and raw memory

Your mission

Implement gm_memchr, your own memchr, which scans the first n bytes of a memory area for a given byte and returns a pointer to the first match, or NULL if it is absent.

See it in action

byte index of gm_memchr("docker", 'r', 6)

Expected result

Your checklist

For your own tracking. Tests grade your code.

Look at exactly n bytes starting at s, one at a time, this works on raw memory, not strings, so never stop early on a \0. A matching byte can sit past an embedded NUL as long as it is within the first n bytes.
Convert c to an unsigned char before comparing: the search byte is the low 8 bits of c, so a value like A + 256 must still match a stored A.
Return a pointer to the first matching byte. Return NULL when no match is found in the first n bytes, including when n is 0.
Do not write to the input buffer, and do not call any library function.

More examples

gm_memchr("zsh prompt", 'X', 10)

Expected result

null

Constraints

Expected files

memchr.c

Allowed functions

None. Write every helper yourself.

Allowed headers

<stddef.h>defines size_t

libc

Standard C library

Implements the contract of a function from the standard C library.

Tests

Run the tests to grade your code

Build your code, then check each test here. Open a result to inspect its output.

Build

01Match at the first byteWaiting

02Match in the middleWaiting

03Match at the last byte within nWaiting

04Byte absent returns NULLWaiting

05Match found past an embedded NULWaiting

06Stops at n, ignores later bytesWaiting

07c is cast to unsigned charWaiting

08Zero length returns NULLWaiting