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POProgram with realloc behave differently in Valgrind
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copied!<p>I wrote a function to read a string with fgets that uses realloc() to make the buffer grow when needed:</p> <pre><code>char * read_string(char * message){ printf("%s", message); size_t buffsize = MIN_BUFFER; char *buffer = malloc(buffsize); if (buffer == NULL) return NULL; char *p; for(p = buffer ; (*p = getchar()) != '\n' && *p != EOF ; ++p) if (p - buffer == buffsize - 1) { buffer = realloc(buffer, buffsize *= 2) ; if (buffer == NULL) return NULL; } *p = 0; p = malloc(p - buffer + 1); if (p == NULL) return NULL; strcpy(p, buffer); free(buffer); return p; } </code></pre> <p>I compiled the program and tried it, and it worked like expected. But when I run it with valgrind, the function returns NULL when the read string is >= MIN_BUFFER and valgrind says:</p> <blockquote> <pre><code>(...) ==18076== Invalid write of size 1 ==18076== at 0x8048895: read_string (programme.c:73) ==18076== by 0x804898E: main (programme.c:96) ==18076== Address 0x41fc02f is 0 bytes after a block of size 7 free'd ==18076== at 0x402BC70: realloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so) ==18076== by 0x8048860: read_string (programme.c:76) (...) ==18076== Warning: silly arg (-48) to malloc() (...) </code></pre> </blockquote> <p>I added a printf statement between *p=0; and p=malloc... and it confirmed that the arg passed had a value of -48. I didn't know that programs don't run the same way when launched alone and with valgrind. Is there something wrong in my code or is it just a valgrind bug?</p>

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